I had the same problem, and did not want to pay to block. So when I get a sales call or call for the previous user, I put it in my Contacts list under the name DNA [for do not answer] and I assigned it the Cricket ring, since my phone does not have a 'no ring' option. So if I hear the cricket chirp, I ignore those. My phone allows 7 or 8 numbers per person, so over time I've created several DNA contacts. Over time, most of them stop calling a number that never answers.